1. Description of the problem
What every clinician needs to know
Relative hyperventilation tending to make blood more alkaline: increased pH, decreased hydrogen ion concentrations
Can be physiological: pregnancy and high altitude
Pathological associated with relative hyperventilation
Often no specific manifestations. Hyperventilation may be apparent.
Key management points
If pathological, treat underlying condition (eg, asthma or cardiac failure).
In ventilated patients, consider decreasing ventilation.
2. Emergency Management
Treat ABC: airway, breathing, circulation.
In symptomatic psychogenic hyperventilation: use rebreathing (paper bag).
In conscious patients acute respiratory alkalosis can cause neurologic symptoms: dizziness, confusion, syncope, seizures, paresthesias (particularly perioral).
Symptoms and signs may be related to underlying conditions such as ischemic or pleuritic chest pain and unrelated to the change in pH. Patients with psychogenic hyperventilation may have symptoms and signs related to severe anxiety.
Normal lab values
PCO2: 35 to 45 mmHg
pH: 7.35 to 7.45
Standard base excess: -3 to 3 mmol/L
Arterial blood gas: pH > 7.45, pCO2 < 35 mmHg, Bicarbonate > 24 mmol/L, standard base-excess < 0 mmol/L.
Compensated metabolic acidosis
Check blood gas results for compensation or second disorder. Metabolic compensation will never be complete (pH < 7.40), and will take hours. Therefore, early respiratory alkalosis may appear uncompensated.
Compensation: Metabolic side compensates for respiratory acidosis by decreasing renal chloride excretion, leading to decreased strong-ion difference and decreased bicarbonate.
Measured by bicarbonate or standard base excess (SBE).
In acute respiratory alkalosis : Expected SBE = 0 mmol/L,
Expected bicarbonate mmol/L = 24+ 0.2 x (PCO2 – 40)
For both bicarbonate and base-excess this estimate is about +/- 2 mmol/L.
Underlying change will be renal chloride excretion leading to decreased strong-ion difference.
In chronic respiratory alkalosis there is adaptation (increase of compensatory effect).
Measured by bicarbonate or SBE.
In chronic respiratory alkalosis: Expected SBE = 0.4 x (PCO2 – 40) mmol/L,
Expected bicarbonate mmol/L = 24 + 0.5 x (PCO2 – 40)
4. Specific Treatment
Often no specific treatment. Outcome depends on underlying disorder.
If psychogenic, consider rebreathing (“brown paper bag”); increases inspired CO2.
In mechanically ventilated patients, consider decreasing minute ventilation.
5. Disease monitoring, follow-up and disposition
Expected response to treatment will depend on underlying problem.
When should I suspect an incorrect diagnosis?
Increased base excess or bicarbonate
Management of underlying condition
Respiratory alkalosis is secondary to relative hyperventilation. The alveolar partial pressure of carbon dioxide, and arterial pCO2, is related to three factors: 1. inspired CO2 (usually zero but can increase in closed environments); 2. body CO2 production; and 3. (inversely) alveolar ventilation.
PCO2 = inspired CO2 + CO2 production / ventilation.
If inspired CO2 is zero this is simplified to PCO2 = CO2 production / ventilation.
The most common cause of decreased PCO2 is an absolute increase in ventilation. Decreased CO2 production without increased ventilation, such as during anesthesia, can also cause respiratory alkalosis. Decreased partial pressure of carbon dioxide will decrease acidity.
Controlled Mechanical Hyperventilation
Drugs: central nervous system stimulants
Common in ventilated patients.
Special considerations for nursing and allied health professionals.
What's the evidence?
Abelow, B. Understanding Acid-Base. 1998. An easy-to-read but comprehensive text that covers both physiology and treatment of acid-base disorders using a bicarbonate-centered view of the non-respiratory side. No mention of base excess.
Gennari, FJ, Adrogue, HJ, Galla, JH, Madias, NE. Acid-base disorders and their treatment. 2005. A more detailed text that highlights some of the clinical chemistry associations of respiratory acid-base changes. Again bicarbonate-centered.
Kellum, JA, Elbers, PWG. Stewart's Textbook of Acid-Base. 2009. A re-issue and extended edition of Stewart's landmark text. Detailed explanation of the Stewart and base-excess approaches.
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- 1. Description of the problem
- 2. Emergency Management
- 3. Diagnosis
- 4. Specific Treatment
- 5. Disease monitoring, follow-up and disposition
- Special considerations for nursing and allied health professionals.
- What's the evidence?